Engines of Our Ingenuity

No.269:
LISTENING DEVICES

by John H. Lienhard

Click here for audio of Episode 269.

Today, we learn acoustics from yesterday's equipment. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Here's an old acoustics book. It came out of England just after WW-II, and it deals with a problem that'd been very much on English minds -- locating enemy airplanes. Radar had matured during the second world war, but it was only starting to emerge from wartime secrecy. The device everyone knew about was an odd, but much simpler, listening machine. It consisted of a large set of mechanical ears riding around on a truck-drawn trailer.

The British sound locator was a brace of four parabolic bowls, about 2-1/2 feet in diameter. They worked best when they received sound at their natural frequency -- 256 Hertz -- just under middle-C. They were highly responsive at middle-C, and increasingly directional at higher pitches.

The Germans, French, and Americans all had their own versions of this technology. American sound locators used arrays of trumpet-like ears -- kind of like speakers on the old hand-cranked phonograph machines.

But airplane speeds rose during WW-II and as they did, sound locators became less and less effective. They could hear an airplane 25 miles away. But they were pure acoustical devices -- not electrical -- it took two minutes for sound to reach them. By the end of the war, airplanes were staying close on the heels of their own noise. Before the airplane noise traveled 25 miles, the airplane itself traveled 16 or 20. By 1945, these acoustical ears gave hardly any warning at all.

The sound mirror was another form of sound locator. A sound mirror was a large partial-sphere with a sensor at its center. It was usually an immovable concrete structure, with a radius of curvature that could reach 150 feet. It worked a lot like a whispering gallery -- focusing sound from 15 miles away onto a central point.

Of course, radar had already made dinosaurs out of these beautiful old gadgets even before this physics book came out. But But they represent a wonderful lesson in practical acoustics. By the time you've threaded through the equations and diagrams, you know a great deal about sound transmission.

A lot of yesterday's technology does that. If you've ever worked on a steam engine, a tube radio, or a Model-T transmission, you've seen their workings spread out before you. Those old machines taught you what no textbook could. Today's printed circuits are far less willing to reveal the human ingenuity that made them. And we're poorer because of it.

I'm John Lienhard at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Poynting, J.H., Thompson, Sir J.J., and Tucker, W.S., A University Text-Book of Physics. Vol. II, Sound, 10th ed., London: Chas. Griffin & Co. Ltd., 1949, pp. 182-207.


The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.
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